Agriculture Reference
In-Depth Information
Table 3.1
Types of crop rotations (Freyer 2003)
F, forage legumes; G, grain legumes; N, non-legumes; C, cover crops; U, undersowings.
No. of
fields
Year
F%
G%
C, U%
1
2
3
4
5
6
7
F
N
N+C
N+U
25
0
50
4
F
N
G
N+U
25
25
25
F
F
N
N+U
50
0
25
F
N
N+C
G
N+U
20
20
40
5
F
F
N
N+C
N+U
40
0
40
F
F
N+U
N+C
N+U
40
0
60
F
F
N
N+C
N+C
N+U
33
0
50
6
F
F
N
N+C
G
N+U
33
17
33
F
F
F
N
N
N+U
50
0
17
F
N
N+C
N+C
N
G
N+U
15
15
42
F
F
N
N+C
N+C
N
N+U
28
0
42
7
F
F
N
N+C
N+C
N+C
N+U
28
0
57
F
F
F
N
N
N+C
N+U
42
0
28
Allelopathic properties of glucosinolate-containing crucifers are highly relevant for their
phytosanitary effects ('biofumigation') against the incidence of soilborne diseases (Lazzari
and Manici 2001, Smith
et al
. 2004a). Buckwheat as green manure may act similarly against
various weeds as a result of the presence of gallic acid (Iqbal
et al.
2003). Infestations by root-
knot nematodes, a severe pest in vegetable cultivation, can be reduced by various green manures
such as radish (Nucifora
et al.
1998), marigold and sudangrass (Abawi and Vogel 2000). Living
mulches can be very effective against various pests as a result of increased predator occurrence
or increased disorientation of herbivores (Hooks
et al.
1998).
Whereas Wijnands (1999) reported very limited use of green manure crops as part of crop
rotations in Dutch organic farms (3 of 68 interviewed farmers), German surveys in organic
agricultural (Rahmann
et al.
2004) and vegetable holdings (Fragstein
et al.
2004) revealed that
60% of arable farms and 63% of vegetable farms used cover crops or undersowing. In a survey
of organic farmers in the USA, the most common (72%) soil fertility management practice
reported was the use of cover crops (Walz 1999).
Designing rotations
Freyer (2003) developed a range of rotations that differ in length and structure, specifically the
proportion of forage legumes, grain legumes and cover or undersown crops (Table 3.1). The
grain legumes should not exceed a proportion of 33%. Forage legumes, if cultivated in mixtures
of clover and/or alfalfa and grass species, can achieve mixtures up to 50%. Because of incom-
patibilities, soilborne diseases and other factors, higher proportions of legumes can cause
serious yield losses in a long-term perspective of rotations (Table 3.2).
The increase of species biodiversity in organic farming is exemplified by the range of poten-
tial cover crops and undersowings used in the rotations. Under favourable conditions forage
legumes can be integrated into the rotation as undersowing in the preceding cereals. If the per-
formance of the undersowing crop is poor, it should be replaced by a stubble crop sown after
cereal harvest. However, the one or multi-year forage legume-part of the rotation is too impor-
tant for the overall success of the rotation (e.g. for N fixation and other tasks) to be able to
accept a 'poor' performance. Cover crops may be best suited when winter crops are substituted
